Interference prevention device



Sept. 4, 1934. R. E, DILLON El AL 1,972,332

INTERFERENCE PREVENTION DEVICE Filed May a, 1928 Patented Sept. 4, 1934 UNITED STATES PATENT OFFICE Robert E. Dillon,

Brookline, Hugo W. H.

Wellington, Dorchester, and Andrew E. Grant,

Wollaston, Mass.

Application May 3, 1928, Serial No. 274,807

5 Claims.

The present invention relates to interference prevention devices for preventing undesired propagation of electrical waves from electrical power stations.

The operation of electrical equipment at generating and sub-stations acts in some instances to generate waves which cause interference with radio reception, not only in the neighborhood of the station itself, but also at distant points which are in proximity to the power line. Such interfering waves are in the nature of transient disturbances and are most frequently the result of the operation of the station switch gear. The opening or closing of a circuit which is connected to the line, even though the circuit itself may be confined to the limits of the station, has been known to set up troublesome high frequency waves which are carried by the line and radiated from it over large areas. The occurrence of the transient waves is believed to arise from the distribution of energy following a. change in the circuit constants caused by the operation of switches at the station. Usually the interfering wave is not confined to a single frequency but extends over so wide a band as to disturb radio reception at all frequencies to which the receivers may be tuned.

It is the object of the present invention to provide means associated with the switch gear of an electrical generating or sub-station which will prevent propagation of interfering waves within the band of frequencies used for radio reception purposes. To this end, the present invention consists in the apparatus hereinafter described and particularly defined in the claims.

In the accompanying drawing, Fig. l-is a wiring diagram of an induction regulator system and its accompanying switch gear embodying the present invention for prevention of interference; Fig. 2 is a wiring diagram of a modified induction regulator system; and Fig. 3 is a detail view of a set of interference preventing inductances.

The present invention, although applicable to any system involving circuits which are opened and closed at the power station, is herein illustrated and described as embodied in means for preventing interference through operation of the automatic regulator circuits of an alternating current supply system.

Referring to Fig. 1, the outgoing three-phase line 4 running outwardly to the left is connected to the. induction regulator 5 at the station. The line is connected by a three-phase transformer 6 with a local three-phase low voltage circuit, having the separate wires 8, 9, and 10. The induction regulator is of usual construction and is operated by a three-phase induction motor 12 in the usual manner. The operation of the motor 12 is controlled to turn the regulator 5 in one direction or the other by means of a contact making voltmeter indicated generally at 14. This contact making voltmeter is of any usual or preferred form, illustrated as comprising the solenoid 16 which is energized by the voltage across one phase of the line through a potential transformer and a compensating impedance of any usual type. The solenoid operates a switch lever 18 which, is adapted to make contact with either the switch point 20 or the switch point 22.

When the line voltage is of the proper value, the lever 18 lies between the two switch points without making engagement with either. If the voltage in the line increases, the solenoid actuates the switch lever to engage with the lower contact 20 and when the voltage falls below its normal value, it permits contact with the switch point 22. Engagement of the switch lever with one of the contacts is availed of to operate an automatic relay which sets the regulator motor 12 in operation, thereby to restore the line voltage to its normal value. To this end, the switch lever 18 is connected to the wire 9 by a wire 24 and the switch point 22 is connected by a wire 26 with a relay coil 28 which in turn is connected to the wire 8. The switch point 20 is connected to the wire 8 through a wire 30 and a relay coil 32.

Energization of one of the relay coils operates the relay switch 34 which is shown as consisting of a double-pole double-throwv switch and which is included in the circuit of the regulator motor. The motor 12 has one phase connected to the line 9 by a wire 36. The other line wires 8 and 10 are connected respectively by wires 38 and 40 with the common or center terminals of the relay switch 34. The upper fixed contacts 42 of the relay switch run, to two phases of the motor 12 by wires 44 and 46 and the lower fixed contacts 48 are similarly connected to the motor but in reverse order by wires 50 and 52, It will be evident that when the relay switch is closed on the upper contacts, the -motor will rotate in one direction, and when a switch is closed on the lower contacts, it will rotate in the opposite direction, thereby operating the induction regulator to restore the line voltage to its proper value.

A limit switch 54 is included in two phases of the induction motor circuit. This limit switch is mechanically operated and is of the usual form designed to open the motor circuit when the induction regulator is rotated to the limit of its movement. The limit switch is shown closed in dotted lines and open in full lines.

The system thus far described is a common type of induction regulator control circuit used in power station work and its operation will be understood by those skilled in the art. The voltmeter switch 18 and the relay switch 34 are operated whenever the line voltage changes from its normal value. The consequent opening and closing of the circuits set up transient oscillations which interfere seriously with radio reception.

In order to prevent transmission of these oscillations along the line, three sets of specially constructed inductance coils are provided. These three sets of coils are indicated generally at 66, 62, and 64. The set comprises a coil in the switch line 24 and similar coils in the wires 26 and 30; The sets of coils 62 and 64 are arranged on opposite sides of the relay switch 34. The set 62' comprises a coil in the main connecting wire scans two coils in the motor con necting wires 4.4 and 50. Similarly, the set ea includes a coil in the main connecting wire 4% and coils in the wires 46 and 52. The three sets of coils are preferably isolated so that no mutual efiects exist between them.

The individual coils of each set may be shielded from one another but are preferably mounted as shown in Fig. 3, each set comprising the three coils indicated at 66, 68, and '79, and wound upon a common support. The coils have an air core and are wound in such a manner as to have as low a distributed capacity as possible. To this end, they are preferably of the honeycomb or universal wound type.

It will be seen that'when a switch is closed, only two of the coils of each set are in circuit. The center coil is always in circuit when a switch is closed and this coil is in series with one of the other coils. For example, considering the set 68 associated with the contact making voltmeter, the center coil in the main wire 24 is in series-with eitherof the outside coils, depending on whether the switch is closed on the contact 20 or the contact 22. Similarly for the sets 62 and 64, the center coil of each is in series with either of the outside coils, depending on whether the relay switch is closed on its upper contacts 42 or its lower contacts 48. Inasmuch as two coils of each set are in series, care should be taken with regard to the polarity in order that the magnetic fluxes which the currents set up may be additive, rather than in such directions as to neutralize each other.

The size of the coils is also of importance. The wire of wh ic'h the coils are wound must be of sufficient size to carry the current required by the circuit. The voltmeter coils 60 which carry small currents maybe wound with fine wire but the coils 62 and 64 must be wound with wire of sunicient size to carry the regulator motor current which is quite high, especially at starting. The inductance of each coil, our experience indicates, should be between .5 and 2 millihenries. In fact, for a three-phase open wire system, it hasbeen found that best results are obtained with inductance: values for each coil ranging between]? and 1' millihenry. For these values, prevention of interference is highly effective for all radio frequencies ranging from the broadcast band to the very high frequencies sometimes employed for radio purposes. vWith lower or' higher values of inductance, the interference increases. The loss of effectiveness is usually evidenced by a narrowing of the band of frequencies within which interference is prevented. This is particularly noticeable as the inductance is increased, values above 2 or 3 millihenries being unsuitable for interference prevention. It is not understood whether this loss of effectiveness with higher values of inductance is due to the inevitable increase in distributed capacity of the coils or to such a change in circuit constants that transient oscillations of interfering frequencies are set up on the line.

It will be seen that with proper values of in ductance, the passage of transient oscillations to the line is eifectively prevented. These transient oscillations ordinarily are set up by operation of the relay switch 34, which carries the heaviest current and to a less degree by the operation of the voltmeter switch 18. The limit switch 54, although it operates infrequently,'is also a source of disturbance because of the fact that it breaks a' circuit carrying a relatively high current. Whatever the source of oscillations may be, there is always at leastone of the inductance coils between the source and the line. If the disturbance arises from a local source of oscillations around the reiay switch, this source is isolated from the line bythe high impedance of the coils. More probably, however, the tendency to set up interfering oscillations resides in the combined constants of the line and the station circuits, and the presence of the inductance coils alters the circuit constants to such an extent that the line is incapable of sustaining oscillations, or such oscillations as maybe set up are outside the band where interference might be caused.

The above circuit is employed where the limit switch is between the regulator motor and the relay switch. In this system, which is a common one in power stations, the leads from the limit switch to the relay switch are quite long and there is a local circuit of considerable length in which transient oscillations may be set up; For this reason, the two sets of coils 62 and 64 are provided in order to provide an impedance between all parts of the local circuit and the three-phase line.

In some cases, it has been found possible to eliminate one set of coils, particularly when the leads from the motor to the relay switch are short. A system of this type is shownin Fig. 2, the shortening of the leads being effected by the arrangement of station equipment whereby the relay switch is placed between the regulator motor and the limit switch. The three-phase line, induction regulator 5, the motor 12 and the contact making voltmeter 14 are the same as before. The relay switch is shown at 134 and the limit switch at 154. The set of coils 162 is connected immediately adjacent to the line. The center coil is in the wire 136 which connects directly to the motor and the outer coils are in the other two wires which connect through the limit switch and the relay switch with the other two phases of the motor. The relay switch operates through energization of the relay coils 28 and 32 to run the motor in one direction or the other, as in the form shown in Fig. 1. The three coils of the set 162 may be separate from each other but it has been found practicable to combine them, as shown in Fig. 3. In this case, however, the threecoils are not in series but are in the separate phases of a three-phase system. The three coils, therefore, carry currents which are 120 out of phase. When this connection is employed, it is necessary to take care that the coils have the proper polarity. It will be seen that if they are all connected in the same relation with respect to the line wires, the magnetic flux will be neutralized except for the leakage flux between the coils. The coils should be connected so that maximum flux is set up and this connection is effected by reversing the phase connection of one of them. The proper connection is best determined by experiment. Any two of the coils may be connected in any desired manner. Of the two possible connections for the third coil, one will prove effective and the other will fail to prevent interference. By a simple test employing a radio receiver in the vicinity of the line, the coils may therefore be connected to give maximum interference prevention.

The invention having been thus described, what is claimed is:

1. An electrical generating system comprising an outgoing line, a regulating circuit including a three-phase regulator motor, a relay switch, means controlled by variation in line voltages for closing the relay switch to operate the regulator motor in one direction or the other, connections from the line to the motor through the relay switch, and an inductance coil in each connection, the three coils being co-axially mounted and mutually inductive, the coils having high impedance to radio frequency oscillations and being of low distributed capacity.

2. The combination with a switch adapted to be connected to an outgoing line and having a movable switch member and two selectively energized contacts, of a set of three inductance coils, co-axially mounted and mutually inductive, of

high impedance to radio frequencies and of low distributed capacity, the center coil being in series with the movable switch member, and the other coils in series with the separate contacts, whereby the center coil is in series with one or the other of the outside coils when the switch is closed on one of the contacts.

3. The combination with outgoing lines of auxiliary circuits associated with the outgoing lines and including regulating and switching equipment, of radio interference prevention devices comprising a set of three inductance coils of low distributed capacity mutually inductive and co-axially mounted, and connected in the separate phases of the auxiliary circuits.

4. The combination with outgoing lines of auxiliary circuits associated with the outgoing lines and including regulating and switching equipment, of radio interference prevention devices comprising a set of three inductance coils of low distributed capacity mutually inductive and coaxially mounted, and. connected in the separate phases of the auxiliary circuits, each coil being of honeycomb type and having an inductance between 0.5 and 2 millihenries.

5. The combination with three-phase electrical lines and a three-phase circuit associated therewith in which current variations occur tending to set up radio interference oscillations, of a universal wound coil in each phase of said circuit, each coil being of low distributed capacity and of an inductance between 0.5 and 2 millihenries to suppress high frequency oscillations.

ROBERT E. DILLON.

HUGO W. H. WELLINGTON.

ANDREW E. GRANT. 

